• Do plants sense their environment?
  • changes in essential resources
  • herbivory/disturbance
  • external stimulus

• If so, what do they do about it?
  • do plants process information?

Cell to cell signal networks

Ca²⁺ based signals are common

• Primary and secondary growth allow trees to be giants

• Are their limitations to plant height?
  • Redwoods = 116 m

• What resource most limits growth?

• Large trees move hundreds of gallons of water a day
  • only small amounts needed for growth/photosynthesis

• Gravity
  • harder and harder to move water up
  • xylem less efficient @ top
  • theoretically, limits growth to 122-130 m

• Drying soil adds opposite tension

• Cohesion tension theory
  • plants use water properties to cheat gravity

• Cohesion: H₂0 stick to other H₂0 molecules
  • via H bonds

• Adhesion: water sticks to non-water molecules
  • stick to sides of xylem tissues
  • H₂0 not pulled straight up!

• Transpiration can pull water under tension

• H₂O also pulled in zig zag motion
  • pits in xylem cells
  • tracheids are twisted

• Vascular tissue is tough
  • high resistance of lignified walls
  • built not to collapse under tension

• Redwoods near theoretical limit!!!